The FAQ on payment options for advertising was updated just 16 hours ago and still lists American Express:https://www.facebook.com/help/212763688755026
Yours is the second question here in the Help Forum I have seen about difficulties with the American Express option--has to be some sort of glitch. Suggest you file a report on the issue--not sure if you have the same report options on a Page as on a personal account: in the same menu as Settings and Log Out is 'Report a Problem' then Something Isn't Working to get to the form--it allows attaching screenshot(s) of the issue.